1. Field of the Invention
The present invention generally relates to a power supply device and an image forming apparatus provided with a power supply device. The present invention particularly relates to a power supply device that detects error when its output and another node are placed in the short-circuit state or open-circuit state, and also relates to an image forming apparatus provided with such a power supply device.
2. Description of the Related Art
It is widely practiced to provide a power supply device with an error detecting unit for detecting an event in which its power supply output node and another node such as a ground node are placed in the short-circuit state or open-circuit state. When the load impedance coupled to the output node exhibits a large change due to a change in external environments such as temperature and/or humidity, however, a given circuit structure may not be able to detect the short-circuit state or open-circuit state correctly.
FIG. 11 is a drawing showing a related-art power supply circuit for the purpose of showing an example of a failure to correctly detect a short-circuit state. The power supply circuit includes transistors Ql and Q2, capacitors C1 through C4, resistors R1 through R5, and transformers 1 and 2. A resistance load 4 is coupled to an output node 3 of the power supply circuit.
In this circuit, the transformers 1 and 2 produce a positive potential and a negative potential, and a power supply voltage is output from the output node 3. A resistor 5 (R3) is provided along an output path extending from the transformer 1 to the resistance load 4. When the impedance of the load 4 becomes significantly smaller than the resistance of the resistor 5, there will be only a slight change between a small load state and a short-circuit state. Because of this, it may not be possible to correctly detect an error.
In an image forming apparatus using an electrophotographic image forming process, an image formed on a photoconductor serving as an image holding member is transferred to an intermediate transfer belt, and is then transferred from the intermediate transfer belt to a paper sheet serving as a print medium. The impedance associated with the intermediate transfer belt tends to exhibit large fluctuations due to changes in temperature and/or humidity. An impedance under a high-temperature and high-humidity condition may exhibit a change by a factor of hundreds in comparison with the impedance under the normal office conditions. A measure to prevent erroneous error detection should thus be taken in such a case. In consideration of this, an arrangement may be made to provide a sensor such as a humidity sensor close to the intermediate transfer belt and to adjust a bias voltage in response to the detected humidity. Japanese Patent Application Publication No. 2003-57889, for example, discloses a technology for use in an image forming apparatus to adjust image forming process control and/or fusing process control in response to temperature and/or humidity conditions.
When a power supply device as previously described is employed in an image forming apparatus, it may be possible to detect a short-circuit state and open-circuit state under normal conditions. It may not be possible, however, to correctly detect a short-circuit state and open-circuit state when a load impedance in the image forming apparatus is changed due to a change in external conditions such as temperature and humidity.
Accordingly, there is a need for a power supply device that can correctly detect a short-circuit state and open-circuit state even when a load impedance is changed due to a change in external conditions such as temperature and humidity.